Pushbutton switches are employed in many types of electronic equipment, including keyboards for typewriters, computers, and other similar devices. In portable electronic equipment, it is usually important to make the keyboard pushbuttons relatively small and arrange to have them closely spaced to one another, and additionally, to have the keys exhibit "low profile" characteristics. A low profile pushbutton switch is one that has a minimal side profile (i.e., its height in the undepressed position) to save space in the vertical dimension. For ergonomic reasons, however, it is preferable that the switches have a full travel stroke in the direction of depression of at least three millimeters (3.0 mm), whether or not such switches are low profile in nature.
In addition to occupying a minimal amount of space on a keyboard (i.e., having a small "footprint"), it is also important that a pushbutton actuator (e.g., used as a key or keyboard switch) does not demonstrate any resistance or binding during actuation. In order to save space, the full travel stroke of some existing low profile key switches is often reduced to only two millimeters (2.0 mm) to save space. In existing pushbutton switches that use vertical sliding bearings, the length of the sliding bearings are also reduced to save space, which means less surface area for the engagement portion of the vertical sliding bearings while in the "up" position, leading to an unstable situation where binding can often occur when the key switches are actuated off-center.
To prevent binding, or to minimize its effects, some of the existing pushbutton switches use some type of stabilizer in an attempt to maintain the upper surface of the switch in the horizontal plane while being depressed. For example, U.S. Pat. No. 5,003,140 (by Abbell Jr., et al.) discloses a stabilizer for a "long" keyboard pushbutton switch. In Abbell, a pair of moveable arms are interconnected by a shaft which runs in a direction parallel to the length of the long keyboard switch. The opposite ends of the arms from the interconnecting shaft are connected to the keyboard pushbutton switch using a thin serpentine section to distribute the stress and force involved as the keyboard pushbutton switch is depressed. The Abbell keyboard pushbutton switch also has a vertical sliding bearing surface to stabilize the keyboard pushbutton switch in one direction while the arms and interconnecting shaft combination stabilize the keyboard pushbutton switch in the other direction (90.degree. from the first direction).
One disadvantage of Abbell is that its design is specifically oriented toward a long keyboard pushbutton switch such as a space bar. If the Abbell design were used in a standard small square keyboard pushbutton switch, its only stabilization in one direction would be via the vertical sliding bearing, which if used in a low profile keyboard, would not have enough surface area to provide the proper stability required for that application. Another disadvantage of Abbell is that its pair of stabilizing arms and interconnecting shaft extend beyond the periphery of the upper cap of the space bar key, and therefore, cannot be used with multiple low profile keys which are to be closely packaged together.
Another patent, by Fleming (U.S. Pat. No. 4,392,037), discloses a pushbutton switch for an electronic keyboard which is stabilized against tilt in two different axes, by use of two anti-roll bars that are attached between the keyboard pushbutton switch and the body of the keyboard. Each anti-roll bar is assembled through lugs along one side of the keyboard pushbutton switch, which is then pressed into position so as to be slidably moveable while held in place by tabs mounted to the upper surface of the keyboard body. The two anti-roll bars are positioned in a 90.degree. orthogonal axis from each other, and from the trajectory of the pushbutton, thereby maintaining the face of the keyboard pushbutton switch normal or perpendicular to the direction of depression. One major disadvantage of the Fleming design is that its two anti-roll bars project away from the sides of the keyboard pushbutton switch, thereby occupying a good deal of space of the keyboard surface. Since the Fleming device was specifically designed for a large area keyboard pushbutton switch (such as the "carriage return" key), the extra area around two of its sides that is used up by the anti-roll bars is not critical. Use of the Fleming design with a large number of closely-spaced keys, however, would be virtually impossible since the anti-roll bars would be in the way of any adjacent closely-spaced keyboard pushbutton switches.
In another patent, by Kato et al. (U.S. Pat. No. 5,120,923), a keyboard pushbutton switch is disclosed having an improved stroke-to-profile ratio. In FIG. 6 herein (which is a partial reproduction of FIG. 13 of Kato et al.) a prior art pushbutton switch is disclosed having a profile "P.sub.1 " which is defined as the distance between the bottom surface of the key cap (in the switch's undepressed position) and the top surface of the membrane switch that the keyboard pushbutton switch is sitting upon. A stroke "S" is defined as the distance traveled from the undepressed position to the depressed position. For an optimal design, each "S" dimension would be equal to each other, otherwise the profile P.sub.1 would be even greater in vertical height than would be necessary.
The stroke-to-profile ratio would be optimally maximized in any keyboard pushbutton switch in situations where it is desired to have a low profile but also to have a large stroke. Since FIG. 6 has a profile P.sub.1 that is equal to (2S)+a.sub.1, its stroke-to-profile S/P.sub.1 ratio is equal to S/((2 S)+a.sub.1. As can be readily discerned from an inspection of this ratio, its value will always be less than 0.5, because dimension "a.sub.1 " is a physical distance greater than zero that is needed for stability of the prior art pushbutton switch of FIG. 6.
FIG. 5 herein (which is a partial reproduction of FIG. 1 of Kato et al.) depicts another embodiment of a keyboard pushbutton switch having a telescoping design along its bearing surfaces. In FIG. 5 the profile is again defined as the distance from the bottom of the key cap to the top surface of the membrane switch that the keyboard pushbutton switch sits upon. This profile dimension can be defined as "P.sub.2 ", which is equal to the dimension (2S)+a.sub.2, in which "a.sub.2 " is the engagement area between the base and the key cap. Dimension a.sub.2 is equal to the upper portion of dimension "E" as depicted in FIG. 5. In the embodiment depicted in FIG. 5, the stroke-to-profile ratio (S/P.sub.2) is equal to S/(2S+a.sub.2). As can be easily discerned from a close inspection of the equation, this ratio must always be a number less than 0.5, since a.sub.2 is a distance greater than zero.
In low profile keyboards used in space-saving applications, such as notebook computers, it is desirable to maximize the stroke-to-profile ratio so that the overall height (or profile) of the keyboard pushbutton switch is minimized while the stroke (the distance between the depressed and undepressed position of the keyboard pushbutton switch's cap) is maintained at an optimal distance of at least three millimeters. In a truly space-saving design, it would be preferable for the stroke-to-profile ratio of a keyboard pushbutton switch to be in the range between 0.5 and the theoretical maximum of 1.0. Since the design of FIG. 5 necessarily has a stroke-to-profile ratio of less than 0.5, it is apparent that its design does not fit within the optimal range.